It may be tough to differentiate botulism from colic, because horses with botulism are often lying down and not eating, and as a result, they may be designated as having colic.

"There have been several occasions, even at a place such as New Bolton Center, where a botulism case was sent to surgery [for colic]," Johnson says. "During surgery, no surgical lesions were found in the gastrointestinal tract. The practitioner then realized, after recovery, that it was botulism."

One clue to distinguish botulism from colic is that horses with botulism usually look worse standing up than lying down, Johnson explains. Additionally, they are often more agitated, have a higher heart rate when standing and have more muscle tremors. When they are lying down, horses with botulism tend to relax. They might not go into lateral recumbency, but even sternal, their muscle tremors stop and their heart rate decreases.

Also, unlike horses with colic, those with botulism don't do "flank watching" and frequently don't go back and forth between lateral and sternal recumbency as frequently as a horse with abdominal pain tends to do.

Another factor that distinguishes botulism from colic is the horse's response to an analgesic, says Johnson. "Giving a horse with abdominal pain an analgesic, such as flunixin, xylazine or detomidine, will usually lead to clinical improvement for the duration of action of the analgesic," Johnson states. "But in horses with botulism, you give them an analgesic and they don't necessarily look better. The tremors continue, and they may still lie down, as it is not affecting the muscle weakness, though it would affect the pain of the colicky horse. This exemplifies another finding to prompt putting botulism on your list."

Other distinguishing signs of botulism

Johnson also notes that once veterinarians have seen a few botulism cases, they're much better at diagnosing it again. Those familiar with the disease will notice additional cranial nerve or subtle deficits, such as weak eyelid tone, dilated pupils, slow pupillary eye reflexes, and weak tail and anal tone. It's also common to notice muscle fasciculations and tremors over the triceps, pectorals or even the hind leg muscles. And although the horse's gait is initially very normal, eventually weakness and a tendency to excessive recumbency will become apparent, says Johnson.

Diagnosing botulism

Although the PCR test, which was developed by Johnson and her colleagues at New Bolton Center, shortens the time to get a definitive diagnosis, she doesn't recommend waiting for the results before treating a symptomatic horse, as it will still be several days for the PCR results to come back. "If you have a botulism suspect case, it should be treated within that interval, rather than waiting," advises Johnson.

It also often helps to confirm the source—and the type—of the toxin. This could be important if someone has recently acquired a large quantity of hay that's possibly contaminated with carrion. "If someone is trying to decide whether to get rid of all that feed, knowing the source of the toxin can help with the decision," Johnson explains.

If a veterinarian knows that a horse has been eating a "risky" food—from a large round bale of hay or one that has been improperly stored, or from a source with known carcass or carrion contamination—it should heighten the suspicion of botulism. Feeding haylage or silage is also considered a risk factor. "Though not a common occurrence, it does happen every once in a while," Johnson says.

There is also a tendency to blame the water as a source of the botulism toxin. "When you have outbreaks with multiple horses involved, it's almost never the water," Johnson explains.